Image acquisition and processing chain for dual-energy radiography using a portable flat panel detector

ABSTRACT

A mobile dual-energy X-ray imaging system is presented. The mobile dual-energy X-ray imaging system is a digital X-ray system that is designed both to acquire original image data and to process the image data to produce an image for viewing. The system has an X-ray source and a portable flat-panel digital X-ray detector. The system is operable to produce a high energy image and low energy image, which may be decomposed to produce a soft tissue image and a bone image for further analysis of the desired anatomy. The system is disposed on a carrier to facilitate transport. The imaging system has an alignment system for facilitating alignment of the flat-panel digital detector with the X-ray source. The imaging system also comprises an anti-scatter grid and an anti-scatter grid registration system for removing artifacts of the anti-scatter grid from images.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Application of U.S. patent applicationSer. No. 11/731,535, entitled “IMAGE ACQUISITION AND PROCESSING CHAINFOR DUAL-ENERGY RADIOGRAPHY USING A PORTABLE FLAT PANEL DETECTOR”, filedMar. 30, 2007, which is herein incorporated by reference in itsentirety.

BACKGROUND

The present disclosure relates generally to dual-energy imaging and, inparticular, to techniques for producing and processing dual-energyimages with a mobile dual-energy imaging system having a portableflat-panel digital detector.

Medical diagnostic and imaging systems are ubiquitous in modern healthcare facilities. Currently, a number of modalities exist for medicaldiagnostic and imaging systems. These include computed tomography (CT)systems, X-ray systems (including both conventional and digital ordigitized imaging systems), magnetic resonance (MR) systems, positronemission tomography (PET) systems, ultrasound systems, nuclear medicinesystems, and so forth. Such systems provide invaluable tools foridentifying, diagnosing and treating physical conditions and greatlyreduce the need for surgical diagnostic intervention. In many instances,these modalities complement one another and offer the physician a rangeof techniques for imaging particular types of tissue, organs,physiological systems, and so forth.

Digital imaging systems are becoming increasingly widespread forproducing digital data that can be reconstructed into usefulradiographic images. In one application of a digital imaging system,radiation from a source is directed toward a subject, typically apatient in a medical diagnostic application, and a portion of theradiation passes through the subject and impacts a detector. The surfaceof the detector converts the radiation to light photons, which aresensed. The detector is divided into an array of discrete pictureelements or pixels, and encodes output signals based upon the quantityor intensity of the radiation impacting each pixel region. Because theradiation intensity is altered as the radiation passes through thesubject, the images reconstructed based upon the output signals mayprovide a projection of tissues and other features similar to thoseavailable through conventional photographic film techniques. In use, thesignals generated at the pixel locations of the detector are sampled anddigitized. The digital values are transmitted to processing circuitrywhere they are filtered, scaled, and further processed to produce theimage data set. The data set may then be used to reconstruct theresulting image, to display the image, such as on a computer monitor, totransfer the image to conventional photographic film, and so forth.

Dual-energy (DE) radiography involves the acquisition of two X-rayimages at different energies within a relatively small time interval.The two images are then used to decompose the imaged anatomy and createsoft-tissue and bone images. Existing digital radiography (DR) imageacquisition and processing techniques were not designed for DEradiography. In addition, the application of DE imaging to mobile DRimaging systems adds several unique challenges. For example, in a mobileDR imaging system, the spatial location of the detector is not alwaysknown relative to the X-ray source, as in a fixed permanent DR imagingsystem. Additionally, the detector may not be mechanically fixedrelative to the X-ray source and may move slightly whenever the patientmoves. As a result, misalignment may occur between the X-ray source andthe detector. Furthermore, mobile DR imaging systems frequently are usedto obtain images of patients that are too sick to move. Consequently,these patients frequently cannot hold their breaths very easily, if atall. As a result, artifacts are created in the image when the lungvolume changes between the first and second exposure.

Accordingly, techniques are needed to overcome the problems associatedwith mobile DR imaging systems. The techniques described herein areintended to solve one or more of the problems associated with mobile DRimaging systems.

BRIEF DESCRIPTION

A mobile dual-energy X-ray imaging system is presented. The mobiledual-energy X-ray imaging system is a digital X-ray system that isdesigned both to acquire original image data and to process the imagedata to produce an image for viewing. The system has an X-ray source anda portable flat-panel digital X-ray detector. The system has awheeled-carrier to enable the system to be transported to a patient. Thesystem is operable to produce a high energy image and low energy imageof a patient, which may be decomposed to produce a soft tissue image anda bone image for further analysis of the desired anatomy. Because of thelimitations of a mobile system in comparison to an installed system, anumber of techniques are utilized to enhance the image acquisition,processing, and display capabilities of the mobile dual-energy X-rayimaging system.

One aspect of the present invention is that the system may utilize arespiratory sensor to perform pulmonary gating during image acquisition.Another aspect of the present invention is a system for facilitatingalignment of the flat-panel digital detector with the X-ray source.Still another aspect of the present invention is a temperaturecorrection function for compensating for temperature gradients in theflat-panel digital X-ray detector after transitioning from a low powercondition to a full power condition. Yet another aspect of the presentinvention is anti-scatter grid registration when an anti-scatter grid isutilized. Additional aspects of the present invention are providedbelow.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a perspective view of a dual-energy X-ray imaging system, inaccordance with an exemplary embodiment of the present technique;

FIG. 2 is a diagrammatical overview of the dual-energy X-ray imagingsystem of FIG. 1, in accordance with an exemplary embodiment of thepresent technique;

FIG. 3 is a diagrammatical representation of the functional circuitry ofa detector of the system of FIG. 1, in accordance with an exemplaryembodiment of the present technique;

FIG. 4 is a perspective view of the detector of FIG. 1 and foam padding,in accordance with an exemplary embodiment of the present technique;

FIG. 5 is a block diagram of an image acquisition and processingtechnique, in accordance with an exemplary embodiment of the presenttechnique;

FIG. 6 is a block diagram of the image acquisition technique of FIG. 5,in accordance with an exemplary embodiment of the present technique;

FIG. 7 is a block diagram of the image pre-processing technique of FIG.5, in accordance with an exemplary embodiment of the present technique;

FIG. 8 is a block diagram of the image post-processing technique of FIG.5, in accordance with an exemplary embodiment of the present technique;and

FIG. 9 is a block diagram of the image display technique of FIG. 5, inaccordance with an exemplary embodiment of the present technique.

DETAILED DESCRIPTION

Although a dual-energy system is described in the foregoing description,the concepts are equally applicable to a multiple energy system as well.Referring generally to FIG. 1, a mobile dual-energy X-ray imaging systemis presented, referenced generally by reference numeral 20. In theillustrated embodiment, the dual-energy X-ray imaging system 20 is adigital X-ray system that is designed both to acquire original imagedata and to process the image data for display in accordance with thepresent techniques. In particular, the system 20 is operable to producea high energy image and low energy image, which may be decomposed toproduce a soft tissue image and a bone image for further analysis of thedesired anatomy.

The mobile dual-energy X-ray imaging system 20 has an X-ray source 22and a portable flat-panel digital X-ray detector 24. The portableflat-panel digital X-ray detector 24 is operable to perform dual-energyX-ray imaging. A patient 26 is located between the X-ray source 22 andthe detector 24. The detector 24 receives X-rays that pass through thepatient 26 and transmits imaging data to a base unit 28. The portableflat-panel digital X-ray detector 24 is coupled by a cable to the baseunit 28 and may be stored in the base unit 28 during transport. The baseunit 28 houses the systems electronics 32 that process the imaging datato produce an image for viewing. In addition, the systems electronics 32both provides and controls power to the X-ray source 22. The power forthe X-ray source 22 is provided by a generator that is operable tosupply the power to the X-ray source 22 to produce both a high energyimage and a low energy image. The base unit 28 also has an operatorworkstation 34 that enables a user to control the operation of thesystem 20 to produce a desired image. Images produced by the systemselectronics 32 are displayed on a display 36. In addition, imagesproduced by the system 20 may be printed onto a film.

The mobile dual-energy X-ray imaging system 20 also includes severalsensors to enhance operation of the system 20. In the illustratedembodiment, a respiratory sensor 38 is provided to send a signalrepresentative of the patient's respiratory cycle to the systemselectronics 32. Because movement of the portable flat-panel digitalX-ray detector 24 is independent of the X-ray source 22, it is possiblefor the X-rays to strike the detector 24 at an angle, producing aninaccurate image of the patient 26. In the illustrated embodiment, thedetector 24 has alignment transmitters 40 that are used to align thedetector 24 with the X-ray source 22 to ensure that the X-rays from theX-ray source 22 strike the detector 24 at the correct angle. In theillustrated embodiment, sensors located proximate to the X-ray source 22are adapted to receive the signals produced by the alignmenttransmitters 40. The system 20 is able to use the signals to triangulatethe orientation and location of the detector 24 relative to the X-raysource 22 to determine if the detector 24 is aligned normal to the pathof X-rays coming from the X-ray source 22. The alignment sensors mayalso be operable to indicate when the detector 24 is within range of theX-ray source 22. When the detector 24 and X-ray source 22 are aligned,an audible and/or a visible indicator are activated. However, theconverse arrangement may be used, i.e., the alignment transmitters 40may be placed on the X-ray source 22 and the sensors within the detector24. In addition, the X-ray source 22 is supported by an adjustable stand42. Finally, the system 20 may be connected to the Internet or othercommunication network so that the images produced by the system 20 maybe sent to a remote user, such as a radiologist's workstation.

Referring generally to FIG. 2, the mobile dual-energy imaging system 20includes a collimator 44 positioned adjacent to the X-ray source 22. Thecollimator 44 permits a stream of radiation 46 to pass into a region inwhich a patient 26 is positioned. A portion of the radiation 48 passesthrough or around the patient 26 and impacts the portable flat-paneldigital X-ray detector 24. As described more fully below, the X-raydetector 24 converts the X-ray photons received on its surface to lowerenergy photons, and subsequently to electric signals, which are acquiredand processed to reconstruct an image of the features within thesubject. FIG. 2 also illustrates the importance of the X-ray source 22and the portable flat-panel digital X-ray detector 24 being inalignment. If not aligned, the portion of the radiation 48 that passesthrough or around the patient 26 cannot be received by the detector 24and an accurate image of the patient 26 cannot be obtained. Furthermore,even if the detector 24 is in the direct line with the X-ray source, thedetector 24 must be angled perpendicular relative to the X-ray source 22for proper detection of the radiation 48.

The X-ray source 22 is controlled by a power supply/control circuit 50,which furnishes both power and control signals for examinationsequences. Moreover, detector 24 is coupled to a detector controller 52,which commands acquisition of the signals generated in the detector 24.The detector controller 52 may also execute various signal processingand filtration functions, such as for initial adjustment of dynamicranges, interleaving of digital image data, and so forth. Both powersupply/control circuit 50 and detector controller 52 are responsive tosignals from a system controller 54. In general, system controller 54commands operation of the imaging system to execute examinationprotocols and to process acquired image data. In the present context,system controller 54 also includes signal processing circuitry,typically based upon a general purpose or application-specific digitalcomputer, associated memory circuitry for storing programs and routinesexecuted by the computer, as well as configuration parameters and imagedata, interface circuits, and so forth. In the illustrated embodiment,the respiratory sensor 38 provides respiratory cycle data to the systemcontroller 54.

The system controller 54 is linked to an output device, such as thedisplay 36 or a printer. The system controller 54 is also linked to theoperator workstation 34 for outputting system parameters, requestingexaminations, viewing images, and so forth. In general, displays,printers, workstations, and similar devices supplied within the systemmay be local to the data acquisition components, or may be remote fromthese components, such as elsewhere within an institution or hospital,or in an entirely different location, linked to the image acquisitionsystem via one or more configurable networks, such as the Internet,virtual private networks, and so forth.

Referring generally to FIG. 3, the functional components of the digitaldetector 24 are presented. In addition, an imaging detector controlleror IDC 56 is presented, which will typically be configured within thedetector controller 52. The IDC 56 includes a CPU or digital signalprocessor, as well as memory circuits for commanding acquisition ofsensed signals from the detector. The IDC 56 is coupled via two-wayfiber optic conductors to detector control circuitry 58 within thedetector 24. The IDC 56 thereby exchanges command signals for image datawithin the detector during operation. The detector control circuitry 58receives DC power from a power source 60. The detector control circuitry58 is configured to originate timing and control commands for row andcolumn drivers used to transmit signals during data acquisition phasesof operation of the system. The detector control circuitry 58 thereforetransmits power and control signals to reference/regulator circuitry 62,and receives digital image pixel data from the reference/regulatorcircuitry 62.

In a present embodiment, the portable flat-panel digital X-ray detector24 consists of a scintillator that converts X-ray photons received onthe detector surface during examinations to lower energy (light)photons. An array of photo detectors then converts the light photons toelectrical signals, which are representative of the number of photons orthe intensity of radiation impacting individual pixel regions of thedetector surface. Readout electronics convert the resulting analogsignals to digital values that can be processed, stored, and displayed,such as on display 36, following reconstruction of the image. In apresent form, the array of photo detectors is formed on a single base ofamorphous silicon. The array elements are organized in rows and columns,with each element consisting of a photodiode and a thin film transistor.The cathode of each diode is connected to the source of the transistor,and the anodes of all diodes are connected to a negative bias voltage.The gates of the transistors in each row are connected together and therow electrodes are connected to the scanning electronics as describedbelow. The drains of the transistors in a column are connected togetherand an electrode of each column is connected to readout electronics.

In the illustrated embodiment, the portable flat-panel digital detector24 has a row bus 64 and a column bus 66. The row bus 64 includes aplurality of conductors for enabling readout from various columns of thedetector, as well as for disabling rows and applying a chargecompensation voltage to selected rows, where desired. The column bus 66includes additional conductors for commanding readout from the columnswhile the rows are sequentially enabled. The row bus 64 is coupled to aseries of row drivers 68, each of which commands enabling of a series ofrows in the detector. Similarly, readout electronics 70 are coupled tocolumn bus 66 for commanding readout of all columns of the detector. Inthe present technique, image acquisition rate is increased by employinga partial readout of the detector 24. In the illustrated embodiment, therow drivers 68 and readout electronics 70 are coupled to a detectorpanel 72 which may be subdivided into a plurality of sections 74. Eachsection 74 is coupled to one of the row drivers 68, and includes anumber of rows. Similarly, each of the readout electronics 70 is coupledto a series of columns. The photodiode and thin film transistorarrangement mentioned above thereby define a series of pixels ordiscrete picture elements 76 which are arranged in rows 78 and columns80. The rows and columns define an image matrix 82, having a height 84and a width 86.

Each pixel 76 is generally defined at a row and column crossing, atwhich a column electrode 88 crosses a row electrode 90. As mentionedabove, a thin film transistor 92 is provided at each crossing locationfor each pixel, as is a photodiode 94. As each row is enabled by rowdrivers 68, signals from each photodiode 94 may be accessed via readoutelectronics 70, and converted to digital signals for subsequentprocessing and image reconstruction. Thus, an entire row of pixels inthe array is controlled simultaneously when the scan line attached tothe gates of all the transistors of pixels on that row is activated.Consequently, each of the pixels in that particular row is connected toa data line, through a switch, which is used by the readout electronicsto restore the charge to the photodiode 94.

It should be noted that as the charge is restored to all the pixels inone row simultaneously by each of the associated dedicated readoutchannels, the readout electronics is converting the measurements fromthe previous row from an analog voltage to a digital value. Furthermore,the readout electronics are transferring the digital values from twoprevious rows to the acquisition subsystem, which will perform someprocessing prior to displaying a diagnostic image on a monitor orwriting it to film. Thus, the read out electronics are performing threefunctions simultaneously: measuring or restoring the charge for thepixels in a particular row, converting the data for pixels in theprevious row, and transferring the converted data for the pixels in atwice previous row.

Referring generally to FIG. 4, an embodiment of the portable flat-paneldigital detector 24 is presented. The detector 24 has an anti-scattergrid 96 that overlays the image matrix 82 of the portable flat-paneldigital detector 24. Scattering is a general physical process wherebysome forms of radiation, such as X-rays, are forced to deviate from astraight trajectory by one or more localized non-uniformities in themedium through which it passes. The anti-scatter grid 96 reduces theeffect of scattering by preventing scattered X-rays from reaching thedetector 24. When using such anti-scatter grids, significantmisalignment between the X-ray source 22 and the grid can result inimage artifacts. The alignment of the detector 24 to the X-ray source 22is hampered by the detector 24 being independent of the X-ray source 22.In addition, because the detector 24 is placed under the region of thepatient 26 to be imaged, the position of the detector 24 is determinedby the position of the patient 26.

To avoid image artifacts from misalignment, transmitters 40 are used toenable alignment of the detector 24 to the X-ray source 22.Alternatively, the alignment transmitters 40 may be disposed on theanti-scatter grid 96 to ensure that the X-rays from the X-ray source 22strike the anti-scatter grid 96 at the correct angle. The X-ray source22 has a receiver that is operable to receive signals from thetransmitters 40 and triangulate the position of the detector 24 relativeto the X-ray source 22. The detector 24 and X-ray source 22 are alignedwhen the detector 24 is positioned so that the plane of the detector 24is perpendicular to the X-ray beam generated by the X-ray source 22 andthe detector 24 is centered relative to the X-ray source. In theillustrated embodiment, the system 20 produces a visual and/or an audileindication when the detector 24 and X-ray source 22 are aligned. Thus,enabling a user to position the detector 24 and ensure that it is inaligned prior to taking an image of the patient 26. In addition, a foampad 98 is placed over the grid 96 in the illustrated embodiment. Thefoam pad 98 creates an air gap between the patient 26 and the detector24 that also reduces the effect of scattering by preventing scatteredX-rays from reaching the detector 24.

Referring generally to FIG. 5, techniques for the processing of imagingdata by the mobile dual-energy X-ray imaging system 20 of FIG. 1 arepresented, and represented generally by reference numeral 100. Certainadaptations have been made in the following techniques because thesystem 20 is mobile. The first techniques in the illustrated embodimentare image acquisition techniques, represented generally by block 102.Once image acquisition is completed, pre-processing techniques areperformed on the acquired image, referenced generally by block 104.After the pre-processing is completed, the acquired images aredecomposed to generate a raw soft-tissue image and a raw bone image,represented generally by reference numeral 106. Next, the acquiredimages are post-processed, represented generally by reference numeral108. Finally, once the post-processing is completed, the acquired imagesare processed for visual display, referenced generally by block 110.

Referring generally to FIG. 6, an exemplary embodiment of the imageacquisition techniques 102 of FIG. 5 is presented. In the illustratedembodiment, the image acquisition techniques 102 include techniqueoptimization techniques for use with a mobile imaging system,represented generally by block 112. Installed X-ray imaging systemstypically have a much larger generator for providing power to an X-raysource. For example, fixed digital radiography systems typically have a60-80 kW generator, whereas mobile systems have generators usually inthe 15-30 kW range. Technique optimization refers to techniques that areused to account for the lower power available for generating X-rays thatexists with a mobile X-ray imaging system in comparison to an installedX-ray imaging system. In the illustrated embodiment, techniqueoptimization 112 includes adjusting the peak kilo-voltage (kVp) andcopper filtration (to harden the X-ray spectrum) relative to aninstalled system.

Once the acquisition parameters are defined, cardiac gating and/orpulmonary gating may be performed, represented generally by referencenumeral 114. Cardiac gating is a technique that triggers the acquisitionof images by detector 24 at a specific point in the cardiac cycle. Thisreduces heart-motion artifacts in views that include the heart, as wellas artifacts indirectly related to heart motion such as lung motion.Cardiac gating addresses lung/heart motion artifacts due to heart/aorticpulsatile motion. Pulmonary gating is a technique that prevents imageartifacts from being created in the image when the lung volume changesbetween the first and second exposure. The lung volume changes can occurwhen the DR imaging system is used to obtain images of patients thatcannot hold their breaths for very long, if at all. In one embodiment ofpulmonary gating, the system 20 acquires both the high energy image andthe low energy image when the lung is at its slowest motion, based onsignals from the respiratory sensor 38 of FIG. 1. In an alternativeembodiment of pulmonary gating, the high energy and low energy imagesare acquired during different respiratory cycles, but at approximatelythe same point in the respiratory cycle.

In the illustrated technique, an alignment of the X-ray source to theportable flat-panel digital detector 24 is performed by a user,represented generally by block 116. As discussed above, the transmitters40 located on the detector 24 are used to align the detector 24 to theX-ray source 22. As discussed above, an audible and/or visibleindication is provided when the detector 24 and X-ray source 22 arealigned. This enables an operator to know that an image taken when theindication is present will have proper alignment of the detector 24 andX-ray source 22.

During image acquisition, an X-ray image is acquired at high energy(“kVp”), represented generally by block 118. In quick succession, anX-ray image is acquired at low energy (“kVp”), represented generally byblocks 120. The low energy image typically is acquired first. The lowenergy exposure may last approximately 100-300 msec. The high energyexposure occurs approximately 0.5 sec later and lasts approximately10-30 msec. The filtration of collimator 44 may be changed in betweenacquisitions to allow for greater separation in x-ray energies. Detectorcorrections may be applied to both the high energy image and low energyimage, respectively. Such detector corrections are known in systemsemploying flat panel detectors and include techniques such as badpixel/line correction, gain map correction, etc., as well as correctionsspecific to dual energy imaging such as laggy pixel corrections. Inaddition, the foam pad 98 described above creates an air gap between thepatient 26 and the detector 24. The air gap improves image acquisitionby reducing scatter.

Referring generally to FIG. 7, an exemplary embodiment of thepre-processing technique 104 of FIG. 5 is presented. The pre-processingtechnique 104 includes a high kVp detector correction, representedgenerally by block 122, and a low kVp detector correction, representedgenerally by block 124. Existing detector correction techniques may beused such as hardware solutions including specialized anti-scattergrids, and or software solutions using convolution-based ordeconvolution-based methods. Additionally, software techniques canutilize information from one image to tune parameters for the otherimage. In addition, detector corrections may be used to compensate forthe effects of temperature on the detector 24.

With fixed X-ray imaging systems, the detector temperature remainsstable once it has initially warmed up. However, mobile X-ray imagingsystems are frequently turned on and off. In addition, the illustratedembodiment of the system 20 has an energy conservation feature, wherebypower to the detector 24 is reduced after a period of non-use.Therefore, the temperature of the detector 24 may not be stable at aquiescent temperature when the mobile X-ray imaging system 20 is used toacquire an image. Typically, when the detector 24 is powered to fullpower, the detector 24 heats up due to the increase in power to thedetector 24. As the detector 24 heats up, a temperature gradient existsspatially across the detector surface, which may affect the pixeloffset/gain and, thereby, affect the resulting image values after anX-ray exposure. A temperature correction function is provided tocompensate for the temperature gradient. The temperature correctionfunction is based on a model of how the detector 24 heats up over timewhen operated at full-power. The input to the correction is the timeinterval between the switch to full power mode and the time of imagecapture. In an alternative embodiment, the detector 24 has a temperaturesensor to provide an input based on the actual detector 24 temperature.The actual detector temperature is then used to establish thetemperature correction function. Noise reduction also is performed. Oneor more noise reduction algorithms are applied to the high kVp and thelow kVp images, represented generally by block 126.

Registration techniques are used to reduce motion artifacts bycorrecting for motion between the high kVp and the low kVp images,represented generally by block 128. The registration algorithms may beknown rigid-body or warping registration routines applied to the highkVp and the low kVp images. The registration processing addressesresidual structures in the soft-tissue image and/or the bone image andlung/heart motion artifacts. In addition, the registration techniques128 include grid registration. When the anti-scatter grid 96 is used,the grid 96 can move independently of the patient 26. The gridregistration corrects for any movement of the grid by aligning the gridsin the high kVp image and the low kVp image.

Referring again to FIG. 5, the decomposition techniques 106 also includeconsiderations based on the dual-energy X-ray imaging system beingmobile. The two images are generally decomposed according to thedual-energy decomposition equations:

IS=IH/IL ^(WS)  (1)

IB=IH/IL ^(WB)  (2)

where IS represents the soft tissue image, IB represents the bone image,IH represents the high-energy image, IL represents the low-energy image,WS is the soft tissue decomposition parameter, WB is the bonedecomposition parameter, and 0<WS<WB<1.

The special considerations for decomposition include using aspatially-variable parameter log subtraction technique that is motivatedby the fact that the decomposition is region dependent under thenon-optimal conditions encountered in a mobile environment. Existing logsubtraction techniques assume that the parameter used for logsubtraction is invariant to a location in the image. However, it hasbeen determined through experience that the parameter is locationdependent. This means that a given value of a parameter works best inone region of the image while a different value of the parameter worksbest in another region of the image. Here, we assume that there are “m”optimal parameters for “m” regions in an image. The spatially-variableparameter log subtraction technique is a method of segmenting theregions into “m” regions, decomposing the image using “m” differentparameters and combining all the results to obtain a combined resultantimage.

The main consideration for the spatially-variable parameterlog-subtraction technique is to modify the log-subtraction parameterbased on the amount of tissue density and put the various parts of thedecomposed image back together in a seamless fashion. In the illustratedembodiment, an algorithm is used to access the high and low power imagepair. The high energy image is segmented into multiple regions based onthe attenuation densities to derive a mask, “M”. The mask is thenprocessed to eliminate any small holes or gaps by changing the region totheir surrounding neighboring regions. The regions are then merged intoa relatively small number of super-regions. For example, thesuper-regions may be divided into high, medium, and low tissuedensities. The images are then decomposed using a standard parameter(“W”) and non-standard parameters (W₁, W₂, W₃ . . . ). This creates anumber of pairs of soft tissue images (IS, IS₁, IS₂ . . . ) and boneimages (IB, IB₁, IB₂ . . . ). Intensity matching is performed for eachof the soft tissue images (IS₁, IS₂ . . . ) to IS to obtain intensitymatched images IS₁, IS₂ . . . The regions in IS are then replaced by theregions in the intensity matched images (IS₁, IS₂ . . . ) guided by themask, “M”. Similarly, intensity matching is performed for each of thebone images (IB₁, IB₂ . . . ) to IB to obtain intensity matched imagesIB₁, IB₂ . . . The regions in IB are then replaced by the regions in theintensity matched images (IB₁, IB₂ . . . ) guided by the mask, “M”. As aresult, the final IS and IB images have seamless, multi-parameter logextracted images.

Referring generally to FIG. 8, an exemplary embodiment of the imagepost-processing techniques 108 of FIG. 5 is presented. Afterdecomposition, a raw soft-tissue image 130 and a raw bone image 132 areproduced. During post-processing, the raw soft-tissue image 130 and theraw bone image 132 are subjected to similar processing techniques. Inthe illustrated embodiment, if the pre-processing techniques 104 did notremove all of the grid artifacts, grid artifact elimination techniques,represented generally by blocks 134, are used to remove any remaininggrid artifacts from the raw soft-tissue image 130 and the raw bone image132. Grid artifact elimination techniques 134 may include frequencynotch filters where the resultant images are analyzed for significantspikes in the spatial frequency domain, which are then suppressed.

A scatter correction technique 136 may be used when the use of ananti-scatter grid is not possible due to clinical considerations. In thescatter correction technique 136, the high gradient edge regions areexcluded from any computations. In the regions that are not in the highgradient edge regions, a weighted average of the resulting image isperformed. In the regions corresponding to the edge regions, values areextrapolated based on the neighboring included regions to create a finalaveraged image. A fraction of the final averaged image is subtractedfrom the original to obtain the scatter corrected image.

Contrast matching 138 is performed match contrast of structures in rawsoft-tissue image 130 and the raw bone image 132 to the correspondingstructures in a standard image. For example, contrast of soft-tissuestructures in raw soft-tissue image 130 (e.g., chest image) is matchedto the contrast in the standard PA image. The contrast matching isperformed to facilitate interpretation of the x-ray images.

One or more noise reduction techniques may be applied to the soft-tissueimage 130 and the bone image 132, represented generally by block 140.The noise reduction techniques 140 address noise due to DEdecomposition. Optional noise reduction algorithms may be neededdepending upon the amount of scatter correction used, especially if ahigh-contrast image is desired. The noise reduction techniques 140 maytune the parameter settings to provide improved visualization of largerstructures and to mitigate the localized, high-frequency noise.

In addition, presentation image processing, represented generally byblock 142, may be performed to the raw soft-tissue image 130 and the rawbone image 132. The presentation image processing 142 includes processessuch as edge enhancement, display window level and window widthadjustments for optimal display. The result of the post-processingtechniques 108 is a processed soft-tissue image 144 and a processed boneimage 146. To allow for timely image review in a clinical environment,image processing can be accelerated by sending the raw images to adedicated offline processor, which would then transmit the processedimage back to the mobile X-ray imaging system 20 for review.

Referring to FIG. 9, an exemplary embodiment of the image displaytechniques 110 of FIG. 5 is presented. The display techniques 110 areintended to cover multiple display techniques including display on amonitor or by a printer. The display techniques 110 include designatingdisplay options and hanging protocols in response to user input (e.g.,radiologists preferences), represented generally by block 148. Thesedisplay options and hanging protocols may be customized or standardizeddepending on the limitations of workstation where the images arereviewed, picture archiving and communication systems (PACS), etc. Forexample, the resolution of the image may be adjusted depending on thedisplay and bandwidth capabilities of the workstation where the imagesare viewed.

Interactive information tools, represented generally by block 150, maybe utilized to make the mobile X-ray imaging system 20 more useful inemergency situations. For example, the interactive information tools 150may provide parameters, such as distance, size, pseudo volume, andobject counts. In addition, the tools 150 may enable a user to draw onthe image, as well as perform stats for regions of interest.

Computer aided diagnosis (CAD) algorithms, represented generally byblock 152, may be applied to one or all of the processed soft-tissueimage 144, the processed bone image 146 and the standard image. The CADalgorithms 152 may be tailored to the processed soft-tissue image andprocessed bone image to improve performance. The processed soft-tissueimage 144 and/or the processed bone image 146, along with the results ofany CAD algorithms are displayed for viewing, represented generally byblock 154. For example, three image types (standard, soft-tissue andbone) may be viewed dynamically on a single display, either in atime-loop or by manual stepping. This visualization technique 154 canpotentially highlight pathologies that are not readily apparent inside-by-side review of images.

Feature-specific enhancement techniques, represented generally by block156, may also be utilized. The mobile X-ray imaging system 20 may beused in situations where surgical and/or monitoring devices are common.In such situations, algorithms that highlight specific devices can helpwith diagnosis and patient management.

In addition, the mobile X-ray imaging system 20 may be connectedwirelessly to a local or a remote workstation. Thus, the images obtainedby the system 20 may be transferred quickly to a radiologist fordiagnosis and treatment.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. An imaging system, comprising: a portable source of radiation; aportable detector operable to detect the radiation from the portablesource, wherein the detector is adapted to move independently of theportable source in all degrees of freedom; and an alignment systemcomprising an indicator, wherein the alignment system is adapted toinitiate the indicator when the detector is aligned with the portablesource to receive radiation from the portable source.
 2. The imagingsystem as recited in claim 1, wherein the indicator is an audibleindicator.
 3. The imaging system as recited in claim 1, wherein theindicator is a visible indicator.
 4. The imaging system as recited inclaim 1, wherein the alignment system comprises a plurality oftransmitters disposed around the portable detector and the alignmentsystem is operable to establish an orientation of the portable detectorrelative to the portable source based on signals from the plurality oftransmitters.
 5. The imaging system as recited in claim 1, wherein thealignment system comprises a plurality of transmitters disposed aroundthe portable source and the alignment system is operable to establish anorientation of the portable source relative to the portable detectorbased on signals from the plurality of transmitters.
 6. The imagingsystem as recited in claim 1, wherein the alignment system comprises aplurality of transmitters disposed around an anti-scatter grid disposedover the portable detector and the alignment system is operable toestablish an orientation of the portable detector relative to theportable source based on signals from the plurality of transmitters. 7.The imaging system as recited in claim 4, wherein the alignment systemcomprises a plurality of sensors disposed around the portable source toreceive signals produced by the plurality of transmitters.
 8. Theimaging system as recited in claim 7, wherein the alignment system isoperable to indicate when the portable detector is within range of theportable source.
 9. An imaging system, comprising: an X-ray source; aportable X-ray detector operable to detect X-rays from the X-ray source;an anti-scatter grid disposed over the detector; and an alignment systemcomprising an indicator, wherein the alignment system is adapted toinitiate the indicator when the detector is aligned with the X-raysource to receive X-rays from the X-ray source.
 10. The imaging systemas recited in claim 9, wherein the indicator is an audible indicator.11. The imaging system as recited in claim 9, wherein the indicator is avisible indicator.
 12. The imaging system as recited in claim 9, whereinthe alignment system comprises a plurality of transmitters disposedaround the portable X-ray detector and the alignment system is operableto establish an orientation of the portable detector relative to theX-ray source based on signals from the plurality of transmitters. 13.The imaging system as recited in claim 9, wherein the alignment systemcomprises a plurality of transmitters disposed around the anti-scattergrid and the alignment system is operable to establish an orientation ofthe portable X-ray detector relative to the X-ray source based onsignals from the plurality of transmitters.
 14. The imaging system asrecited in claim 12, wherein the alignment system comprises a receiverattached to the X-ray source to receive signals produced by theplurality of transmitters.
 15. The imaging system as recited in claim13, wherein the alignment system comprises a receiver attached to theX-ray source to receive signals produced by the plurality oftransmitters.
 16. The imaging system as recited in claim 9, comprising afoam pad disposed over the anti-scatter grid to prevent scattered X-raysfrom reaching the portable X-ray detector.
 17. An imaging system,comprising: a portable source of radiation; a portable detector operableto detect the radiation from the portable source, wherein the detectoris adapted to move independently of the portable source in all degreesof freedom; an alignment system comprising an indicator, wherein thealignment system is adapted to initiate the indicator when the detectoris aligned with the portable source to receive radiation from theportable source; and wherein the alignment system comprises a pluralityof transmitters disposed around the portable detector and/or ananti-scatter grid disposed on the portable detector and the alignmentsystem is operable to establish an orientation of the portable detectorrelative to the portable source based on signals from the plurality oftransmitters.
 18. The imaging system as recited in claim 17, wherein theindicator is an audible indicator.
 19. The imaging system as recited inclaim 17, wherein the indicator is a visible indicator.
 20. The imagingsystem as recited in claim 17, wherein the alignment system comprises aplurality of sensors disposed around the portable source to receivesignals produced by the plurality of transmitters.